1. Field of the Invention
This invention relates to an exhaust control system having a catalytic device in an exhaust line for purifying exhaust gases.
2. Description of Related Art
In an internal combustion engine of an automotive vehicle, a catalytic device, such as a catalytic converter, is typically disposed in an exhaust passage for purifying exhaust gases discharged from the vehicle through the exhaust passage. It is desirable to promptly activate the catalytic device after the engine starts so that its inherent gas purifying function can be effectively performed. Therefore, the catalytic device is provided with a heater which receives electric power from an electric power supply of the vehicle and heats the catalytic device in order to promptly activate the catalyst of the catalytic device. In a vehicle equipped with an engine which has an exhaust system including such a catalytic device and a heater, a large amount of electric power is conventionally needed for the heater. Accordingly, a large battery having a large capacity must be installed in an engine compartment or the like. Such a large size battery may cause a shortage of space for various necessary elements which must be installed in the engine compartment.
In some engines of this kind, a primary catalytic device, disposed in the exhaust passage, and a secondary catalytic device, which has a smaller capacity than the primary catalytic device, disposed in the exhaust passage upstream from the primary catalytic device, are provided. When the engine cranks after it starts at a low temperature, the secondary catalytic device is initially heated by exhaust gas to its active temperature so as to purify the exhaust gas. The exhaust gas, purified and heated by the secondary catalytic device, is then led to the primary catalytic device, and promotes activation of the primary catalytic device. As a result, a time period before the primary catalytic device is heated and activated sufficiently to purify the exhaust gas is shortened. Such an engine and catalytic device arrangement is known from, for instance, Japanese Unexamined Patent Publication No. 2-256,815.
Even in an exhaust system which has a primary catalytic device and a secondary catalytic device which has a smaller capacity than the primary catalytic device and is disposed upstream from the primary catalytic device, exhaust gas is allowed to be discharged to the outside of the vehicle without being purified by both the secondary catalytic device and the primary catalytic device until the secondary catalytic device reaches its active temperature during warming up of the engine. Such a problem also occurs even in an exhaust system which has a heater for the primary catalytic device.